Usefulness and Limitations of Polymerase Chain Reaction (PCR) for the Diagnosis and Management of Toxoplasmosis Following Allogeneic Hematopoietic Cell Transplant: Single-center Experience With 31 Patients Over 16 Years.

Background: Toxoplasmosis is an early post-transplant complication in recipients of allogeneic hematopoietic cell transplant (HCT), typically arising from reactivation of latent infection. polymerase chain reaction (PCR) has improved detection.

Methods: Single-center, retrospective review of allogeneic HCT recipients who developed toxoplasmosis from August 2008 to November 2024.

Results: We identified 31 cases of toxoplasmosis among 1235 HCT recipients. Ten had infection and 21 had end-organ disease. Fever was the most common clinical manifestation (74.2%). Patients with pulmonary or central nervous system disease often lacked organ-specific symptoms. Toxoplasmosis primarily occurred in patients not on prophylaxis (90.3%), at a median of 28 days post-HCT (interquartile range 20-69 days). Whole blood PCR diagnosed 80.6% cases and showed a cumulative sensitivity of 93.3%. However, PCR was not always positive at symptom onset, and some asymptomatic patients already had end-organ disease at the time of first PCR positivity. Trimethoprim-sulfamethoxazole (TMP-SMX) was the most used treatment (48.4%). Mortality directly attributable to toxoplasmosis was 12.9%, but all-cause mortality was 61.3%.

Conclusions: Toxoplasmosis is an early post-HCT complication with high morbidity and mortality. Prophylaxis is essential. TMP-SMX is effective, but sometimes it is withheld early post-HCT due to potential myelotoxicity. Given the short window between infection and progression to disease, we recommend twice-weekly monitoring with whole blood PCR while off TMP-SMX and early initiation of TMP-SMX post-HCT for seropositive patients. Atovaquone may be considered as a bridging prophylaxis until TMP-SMX is started, but its absorption may be compromised early post-HCT and breakthrough cases have been reported.